1. Technical Field
The present application is directed to a steering device for vehicles, and in particular to a steering device comprising a steering shaft, a sensor for determining the movement of the steering shaft, and a circuit for evaluating the measuring signals of the sensor.
2. Background of Related Art
Vehicle steering mechanisms may take different forms. Rack and pinion steerage is used particularly often. With rack steerage, a driver exerts a torque on a steering column via a steering wheel. Direct power transmission then continues via a pinion, i.e. a gear wheel, to a rack. Longitudinal movement of the rack results in longitudinal movement of a steering shaft in, or on which, the rack is mounted. The steering shaft in turn moves the steering gear, with the vehicle wheels arranged on it, and is steered in this manner.
To assist the direct power transmission by the driver it is also known to use hydraulic power-assisted steering mechanisms, in which a pressure chamber runs a piston fixed to the steering shaft. By controlling the pressure in the chamber filled with hydraulic oil the piston can be moved, thereby assisting the steering gear in addition to the power transmission by the driver. Alternatively, the pinion drive may be assisted by an electric motor.
In order to provide these various forms of assistance it is naturally desirable to have a measuring signal available which correlates with the state of the steerage. The signal could then take over appropriate control to boost the steering, for power-assisted steering and similar purposes, and could also allow for self-regulating systems. Over and above the control of the servo mechanism, allowance should also be made for boosting measures to optimise the steering and attenuation action of motor vehicles or simultaneous control of all four wheels and other intelligent steering systems.
Various proposals have already been made for obtaining a signal which correlates with the state of the steerage.
Thus, it is proposed in DE 40 29 764 A1 to arrange length measuring means between the steering wheel and the front axle, responding to displacement of the steering rack. Inductive or ohmic devices are proposed for these means. A design with two magneto-resistive sensors is known from EP 0 410 583 B1. Here, the magnetic coupling is changed on movement of the steering shaft, thus enabling the position to be determined. However, this involves changing the geometry of the steering shaft and also providing it with a groove, which apart from the expense, gives it a certain susceptibility to trouble. EP 0 376 456 B1 also operates with a magnet which is arranged on the steering shaft and surrounded by an induction coil. A change in induction can be associated with a change in displacement.
Steering angle sensors operating with magnetic field sensors, so-called Hall sensors, are known from DE 197 03 903 A1 and DE 197 52 346 A1.
These known proposals have the drawback that measurement only allows restricted accuracy. Another problematic feature is that the measurements are relative, so that measuring errors add up over time. The proposals are not, therefore, practicable for use in intelligent steering systems.
It is known from DE 37 03 591 C2, in a rack steering mechanism at the end of the steering column, to measure the rotary angle of the column by appropriately acting on an induction coil or a piezo power-measuring cell. However, the end of the steering column also carries the power transmission to the steering rack and is both structurally confined and unfavourable for measurements, particularly as a great deal of malfunctioning may take place there.
There is, therefore, needed in the art a steering device in which it is possible to pick up a signal correlating with the state of the steering mechanism and more suitable for controlling intelligent steering systems of that type.